Attention is focused on an organic EL element as a display device, because high luminance light emission can be produced at low voltage. The organic electroluminescent element is constituted in such a manner that an emission layer containing a light-emitting compound is sandwiched between a cathode and an anode. In the above element, electrons and holes are injected into the emission layer and are subjected to recombination, whereby exciton is generated. The foregoing organic electroluminescent element is an element from which light is emitted by utilizing light emission (fluorescence or phosphorescence) during deactivation of the resulting exciton.
A demand for a surface light-emitting element in small volume and low power consumption has been desired, and an electroluminescent element (hereinafter, abbreviated as “EL element”) draws attention as one of the surface light-emitting elements.
And, the EL element is classified broadly into an inorganic electroluminescent element (“inorganic EL element”) and an organic electroluminescent element (“organic EL element”), depending on the material constituting such an EL element.
Generally, the inorganic EL element emits light in such way that high electric field is acted to a light-emitting section, and electrons are accelerated in the high electric field to come into collision to emission center, whereby the light emission center is activated to emit light.
On the other hand, in an organic EL element, electrons and holes are respectively injected into a light emission layer from an electron injection electrode and a hole injection electrode, the organic material is raised to excited state via combination of the injected electrons and holes in the light emission layer, and light is emitted when the organic material come back from the excited state to the ground state. The organic EL element has an advantage, which is capable of lower voltage driving than that of the inorganic organic EL element.
It is expected to develop to a thin and flexible display and lighting usage utilizing advantage of surface light emission. In this case, a white light organic EL element is expected to be developed for back light of a liquid crystal display or lighting.
Several methods of constituting a light emission layer to produce the white light organic EL element are known. A method of preparing the light emission layer composed of plural layers each containing each of BGR emitting materials, and a method of preparing a single light emission layer containing BGR emitting materials are known.
Further, the element is subjected to an aging treatment to reduce fluctuation among organic EL elements, and a trial to obtain longer lifetime has been made (refer to Patent Documents 1 and 2).
However, there appears a problem such that even when in a white organic EL element possessing a light emission layer, annealing treatment conditions to stabilize luminance and color during aging are slightly different from each other, an organic EL element exhibiting stable performance can not be obtained, and fluctuation is produced among organic EL elements, whereby no organic EL element exhibiting stable performance can be prepared.